Genistein and daidzein are estrogenic isoflavones found in soybeans. Although they are much less potent than estradiol, infants fed soy formula have serum concentrations that are up to 4 orders of magnitude higher than estradiol. We have been in the field with a longitudinal study of infants up to 9 months old, looking in detail at the growth and development of their estrogen-responsive tissue as well as sex hormones and gonadotropins. We are just starting a study of one- and two-year olds, looking at bone growth, play behavior, and language acquisition. For all these studies, the exposed group are children using soy formula, compared with breast fed or cow milk formula. We also used data from a longitudinal study in England that evaluated behavior and language, and also was begun long enough ago to have data on menarche in girls. We found that girls fed soy early had play behavior intermediate between girls fed cow milk and boys;we also found that girls given soy early reached menarche a few months earlier.